Considerable demand exists for cation-exchange resins capable of selectively removing metallic ions from solution. One field of application is the area of hydrometallurgy, wherein it is the object to prepare concentrated solutions of the desired metal (e.g., copper, cobalt, nickel, or zinc) or in the extraction of precious metals from solution (such as gold, silver, or platinum). Such resins also find utility in the removal of impurities from electroplating baths, the regeneration of acids used in metal stripping, and the purification of water and various aqueous wastes.
While conventional cation-exchange resins remove metallic ions from aqueous solution quite readily, their use is limited since they act relatively nonselectively, thus requiring a succession of elution steps with suitable chemical reagents to obtain the desired metal.
A number of selective cation-exchange resins have been proposed. For example, U.S. Pat. No. 2,888,441 and 2,875,162 describe cross-linked polymers having alpha-amino carboxylic acid groups. In U.S. Pat. No. 3,345,344 the preparation of high molecular weight resins containing polyhydroxamic acid groupings from polyamidoximes is described. Further, the preparation of various condensation products, such as hydroxyquinoline-formaldehyde, resorcinol-formaldehyde, and salicylic acid-formaldehyde is known.
Such resins also have some limitations. In the case of the addition polymer materials, good mechanical and chemical stability are found, but a generally insufficient exchange rate is noted. In the case of condensation products, mechanical-chemical stability is questionable and, in addition, the process may often result in modification of the chelating or complexing groups.